The invention relates to an input circuit for a signal from an inductive speed sensor.
Many internal combustion engines use transmitter wheels with inductive sensors to determine, for example, the position of the crankshaft. Not only are inductive sensors of this type rugged and usable at very high temperatures, they are also extremely inexpensive. However, the amplitude of the signal depends on the speed, covering a range from a few millivolts to more than 100 volts. Electronic circuits are normally used in order to be able to detect low amplitudes at low speeds, on the one hand, while reaching, on the other hand, the highest possible level of noise immunity during normal engine operation, i.e. in the presence of high amplitudes. The electronic circuits either divide the existing sensor voltage in one or more stages, or they switch the switching thresholds of evaluating comparators over. Both of these methods measure the average level of the sensor voltage and effect the switch-over of their evaluation dependent on the latter.
A disadvantage of this process is that it necessarily involves a relatively high level of complex circuitry in order to determine the average amplitude of the signal, to switch-over the thresholds or voltage dividers, to provide a hysteresis for the switch-over and to prevent undesirable additional edges that may occur in the more sensitive area during a switch-back. Therefore, specially designed and costly so-called ASICs are often used.
One object of the present invention is to provide an input circuit of the kind described at the outset that can be used to achieve a high degree of input sensitivity during the start-up phase and a good signal-to-noise ratio during normal engine operation utilizing the simplest means.
This and other objects and advantages are achieved by the input circuit according to the invention, in which a voltage divider for a signal amplitude is no longer switched over on the basis of an average sensor signal value but on the basis of the speed. A precise analysis of the above-mentioned problem revealed that the low amplitudes occur, for the most part, only during the start-up process (that is, when the engine starter rotates at less than 100 rpm). But once the engine starts, the idling speed is reached within a very short time. Idling speeds, however, are within a range of approximately 500 to 1,000 rpm. At this speed, the amplitude of the transmitter signal reached approximately 10 times the initial amplitude. The amplitude, in turn, changes by a maximum factor of ten across the entire remaining speed range. The invention takes advantage of the dependence of the signal amplitude on speed.
Specifically, a micro-controller can provide the speed to the engine control, where this dimension is already present. The speed thresholds and the switching hysteresis are also easily adjustable with the present invention.
Furthermore, if taking into account that the initial sensitivity level is necessary only during the start-up phase, a simple switch-over threshold will be enough to ensure sufficient noise immunity during normal engine operation.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.